<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <style>
        #outter{
            border: solid 1px green;
            display: flex;
            flex-direction: column;
        }
        .item{
            height: 100px;
            width: 100px;
            border: solid 1px red;
            float: left;
        }
    </style>
    <title>Document</title>
</head>
<body>
    <div id="outter">
        <div class="item">1</div>
        <div class="item">2</div>
    </div>
</body>
</html>